This invention relates generally to fiber-optic, evanescent-wave biosensors and, in particular, to improved cell design for such sensors.
Optical fibers are being used in a variety of biosensor applications. For example, as discussed in U.S. Pat. No. 5,494,798, an optical fiber may be used without cladding to exploit the evanescent field present immediately outside the fiber/air interface. If a monoclonal or polyclonal antibody is attached to the exposed surface of the bare fiber, the evanescent field envelopes the molecule. But since there is little or no absorption or other phenomena to alter the amount of the light carried by the fiber, no attenuation or detectable characteristics are developed.
However, when an appropriately labeled antigen is attached to the antibody, the evanescent field can cause the antigen to fluoresce, resulting in an optically detectable characteristic such as a reduction in light intensity or fluorescence. Alternatively, by first binding the antigen, the sensor can be used to detect unknown targets, including toxins or immunogenic agents.
Whereas previous fiber-optic evanescent-wave sensors utilized multi-mode fibers, the ""798 patent improved on the technique by employing a pair of single-mode optical fibers in a coupler arrangement. Light is introduced into one of the fibers to produce an evanescent region surrounding the coupling area, and the magnitude of light emitted from the pair of fibers is compared for detection purposes.
FIG. 1, taken from the ""798 patent, shows the overall fiber optic system generally at 10. Light from laser diode 14 is inserted into a first leg 17 of a fiber optic coupler 18, and exits on the same fiber at 19 (input channel). A second fiber 20 provides an output channel for light from the first leg 17. A first photo diode detector 21 is connected to the input channel and a second photo diode detector 22 is connected to the output channel.
Each detector feeds its own transimpedance amplifier. The outputs of the transimpedance amplifiers 23, 24 are applied to A/D converters 25 and 26 which provide digital electrical signals along wires 27 and 28 to an instrumentation board 29. The instrumentation board 29 is then connected to a personal computer 30 which provides outputs to a printer or a monitor.
The finished probe includes the coupler and attached antibodies, which yields a baseline ratio for the sensor. The finished probe is then exposed to a material of interest, and the ratio of the light through the two sides of the coupler changes as a function of the way in which the target attaches. That is, the localized index of refraction at the coupling region and the determination of the ratio is a function of the binding in the coupler region.
In terms of the coupler itself, existing designs use off-the-shelf components intended for multiplexers and demultiplexers in telecommunications applications. Corning, for instance, makes these couplers by twisting together two or more 1300-nm, single-mode type 9-125 optical fibers, heating up the twisted area and pulling the ends apart to create a necked-down, nearly fused union. The number of fibers and other factors such as the proportion of each fiber in the twisted region determines the coupling ratio.
FIG. 2 depicts a typical commercially available cell. The device includes a central coupler section 202, about 4 inches long and xc2xc-inch in diameter, from which leads 204 emerge from either end. The total length is on the order of 18xe2x80x3 or thereabouts. Often in multiple cells of this kind must be interconnected, in arrays, trees and other configurations. As the number of interconnected cells grows, the layout can become unwieldy. The need remains, therefore, for a more organized way of interconnecting multiple optical couplers, regardless of the end application.
This invention resides in an improved biosensor cell of the type used with a source of light such as a laser and an optical detector operative to sense changes in the light which might be indicative of a chemical or biological material. The apparatus comprises a fluid-carrying chamber and a fixture configured to receive the chamber. The chamber includes one or more optical waveguides immersed in the fluid, each waveguide having an input end and an output end, both of which are optically accessible from outside the chamber. The fixture includes a first optical path for routing the source of light to one end of one of the optical waveguides, and a second optical path for routing the other end of the optical waveguide to the optical detector, such that the fluid-carrying chamber may be removed and replaced with the alignment of the ends of the waveguide and the optical paths being physically maintained.
In the preferred embodiment the waveguide is a fiber-optic coupler having a necked-down, fused region generating an evanescent field that extends into the fluid. The fluid-carrying chamber preferably includes an inlet and outlet to establish a flow around the optical waveguide. A chemical or biological constituent is disposed on the necked-down region within the evanescent field, such that binding alters the light exiting from the coupler for detection purposes. However, the invention is not limited in terms of the optical waveguide, in that single fibers, capillaries, integrated optical circuits and other conduits may be used.
The preferred embodiment also includes a plurality of optical waveguides, with partitions to establish a serpentine path around the waveguides for comprehensive exposure to the fluid. The fixture is also preferably configured to simultaneously receive a plurality of the fluid-carrying chambers, whether in a planar 1-by-X array or stacked to achieve an X-Y configuration.